The present invention generally relates to wireless networks, and more particularly to a method for transmitting synchronization information within a multi-hop network to enable the nodes in the network to acquire and maintain synchronization with a master node.
Many wireless networks require the included nodes to be time synchronized with one another. Some require this synchronization for the purpose of controlling access to a shared communication medium, some for coordinated transfer of global information, and for others, it is required for synchronous and time-stamping applications in the network. A central controller broadcasting synchronization information to all its nodes typically accomplishes the synchronization. In this case, all the nodes become synchronized with the clock residing in the central controller, i.e. the master clock. An example of this type of system would be a cellular system where the base station is the central controller and the cell phones are the nodes.
Another example is an IEEE 802.11 wireless infrastructure network, where the access point is the central controller and the wireless stations are the nodes. This method of synchronization works well when all the nodes of the network are in the transmission range of the central controller.
Another type of network that requires synchronization among its nodes to operate properly is the IEEE 802.11 ad-hoc network. However, this network does not have a master clock. In this network, the nodes receive synchronization information via beacon packets from different nodes at different times. Each node defines a series of transmission times called Target Beacon Transmission Times (TBTT) that are Beacon-Period time units apart, where a Beacon-Period is a parameter of the network. At each TBTT, a node would listen for a beacon at a random interval and then transmit its own beacon if none arrived at the end of this interval.
On the other hand, if a beacon arrived before this random interval ends, the node would refrain from transmitting a beacon and update its local timer (or clock) with the received beacon. This process is repeated at each TBTT. In this type of network, all the nodes share responsibilities in acquiring and maintaining synchronization. Although, all the nodes in the ad-hoc network do not have to be within range of each other, the synchronization algorithm described above would work best when all the nodes are within range of each other.
Another type of network that also requires synchronization is a wireless multi-hop network with a master clock. The central controller based networks described above are not multi-hop but single-hop networks. A wireless multi-hop network is one where some or most nodes are not within the range of the node with the master clock (referred to as the xe2x80x9croot nodexe2x80x9d). In other words, some nodes are more than one xe2x80x9chopxe2x80x9d away from the root. Therefore, in order to achieve synchronization, some nodes must receive synchronization information from nodes other than the root.
An example of a multi-hop network with a master clock is a wireless version of an IEEE 1394 network where some of the wireless nodes are not in the range of the root, as described in the specification of a wired IEEE 1394 network. In order to operate properly, the local timer of all the nodes in an IEEE 1394 network must be synchronized to the timer of the root.
In a wireless 1394 network, the nodes may not be within the transmission range of all other nodes for a number of possible reasons. One obvious reason is the area coverage of the whole network is larger than the range of each node. Another reason is that the power or range of the nodes is limited intentionally to increase the overall capacity of the network and/or to reduce interference to other wireless devices in the vicinity. In this regard, the nodes in a wireless multi-hop network must acquire and maintain synchronization with a master clock.
The present invention is directed to a method of selecting nodes in a multi-hop network having a combined coverage that reaches all the nodes in the network. These nodes, called parent nodes, together with the root node transmit synchronization information regularly to the rest of the network. As part of the selection process, the parent nodes that would not interfere with each other are identified and then scheduled to transmit synchronization information in a number of channels. The parent nodes that are a predetermined number of hops from each other transmit in the same channel.
Further, a time division structure can be used to control the access of these parent nodes to a common communication channel, resulting in the efficient use of the bandwidth available. The time division structure also ensures that each node receives regular synchronization information so that it can acquire and maintain synchronization with the root node. Minimal coordination between the nodes is required to implement this method and timing structure. Further, other structures are also contemplated such as frequency division, code division and hybrids.